1. Field of the Invention
This invention relates to a mold additive for use in a continuous casting (hereinafter referred to as mold powder), and more particularly to a mold powder useful for the application to steels of a type having a low hot strength. That is, the invention is concerned with a useful mold powder which can provide cast slabs having excellent surface properties without causing the casting troubles such as breakout and so on even under a high-speed casting or high cycle mold oscillation condition.
2. Description of the Prior Art
In the continuous casting using the same mold powder, when the casting is carried out at a higher speed or under a higher cycle mold oscillation, the consumption of the mold powder (i.e. amount of slag flowing from a meniscus in the mold into the gap between the mold and the solidification shell) is reduced to damage the lubrication inside the mold, which tends to cause the breakout or degrade the surface properties of the resulting cast slab. In order to increase the consumption of the mold powder, it is usually better to use a mold powder having low viscosity and crystallization temperature. Even if such a mold powder is used, however, the improvement on the surface properties of the cast slab is insufficient by experience though the consumption is increased, so that it is required to take another countermeasure.
For instance, in order to apply a cast slab of ferrite stainless steel (SUS 430) to a rolling without scarfing, it is indispensable to eliminate defects of the cast slab by lightening the formation of oscillation mark and preventing the occurrence and catch of slag inclusion in the oscillation mark portion and the formation of positive segregation. For this purpose, the casting is required to be carried out under a high cycle mold oscillation condition of not less than 150 cpm, preferably not less than 180 cpm.
In the ferritic stainless steel, however, the hot strength is low as compared with the other types of steel, so that the strength of solidification shell in the resulting cast slab is small and hence the degree of thrusting the solidification shell to the inner wall surface of the mold by static pressure of molten steel inside the cast slab becomes large. As a result, the gap between the mold and the solidification shell becomes smaller, which is apt to obstruct the flowing of the mold powder and hence cause a restraining breakout called as a sticking. Under the above circumstances, there is substantially no case that the steel of this type is stably cast at a cycle number of not less than 130 cpm up to now.
Therefore, a cast slab of SUS 430 (200.times.1,260 mm) was cast at a drawing speed of 0.9 m/min and a mold oscillation number of 210 cpm by using a mold powder shown as Comparative Example I in the following Table 4. In this case, the consumption of the mold powder is increased to 0.40 kg/t, which exceeds 0.35 kg/t being an empirically confirmed threshold consumption on the occurrence of breakout. However, the solidified steel cluster called as "Deckel" is formed on the surface of molten steel in the mold, which is considered to be caused by the heat of decomposition of excessive carbonate. Further, not only a large number of slag inclusions but also fine longitudinal cracks with a length of several tens millimeters are produced in the surface of the slab.